The prior art includes a number of video memory systems, using a variety of memory organization schemes. The present invention is directed at multiple station computer systems which use bit mapped displays.
For those not skilled in the art, a "bit mapped" video display system is a display system in which the picture for a display is stored in a memory array, with a separate bit (or set of bits) of memory being used to store the picture for each position on the system's display. Every position on the display is called a pixel, and a typical display has 400 horizontal lines of 640 pixels.
A basic monochrome (i.e., black and white) display system generally needs only one bit of memory for each pixel because the pixel is either on or off. Many so-called monochrome displays have three levels of illumination for each pixel: dark, on, and bright. Such systems require two bits of memory per pixel. Other monochrome display systems have additional levels of shading, requiring additional bits of memory for the storage of each pixel, and are sometimes called gray scale display systems. For the purposes of this discussion, the term "monochrome display" is defined to include all of these types of non-color display systems.
A color display system typically needs at least four bits of memory for each pixel so that at least sixteen different colors can be used. Clearly, the more colors used the greater the number of bits needed per pixel. Another aspect of most color display systems is that a "color map" is used to define what color corresponds to each binary pixel value stored in memory. This color map is generally a high speed memory array which translates the video data for each pixel into the signals needed by the system's video signal generator.
In the prior art bit mapped video display systems known to the inventors, each display either has its own frame buffer or shares a memory array with a data processing unit. The present invention differs from these prior art systems in that a single memory buffer is used to hold the video data for a plurality of displays. In the preferred embodiment, a preselected portion of this memory buffer is allocated for general memory usage by the system's data processing unit, and the rest of the buffer is used for storing video data.
In some prior art memory systems, the portion of the system's memory array which is used as a frame buffer can either be a predefined portion of the array or can be dynamically allocated. In fact, a large amount of effort has been devoted to the design of dynamic memory allocations systems. Dynamic allocation systems are useful for a number of functions including scrolling of the screen and allowing switching between two or more screens of data without having to pause while the video memory is rewritten. The present invention, however, is directed at a different type of improvement, specifically an improved memory system for multiple video displays.
One shortcoming of the aforementioned prior art is that in multiple station computer systems, the memory control hardware is repeated for each station. Since video memory systems have fairly stringent timing constraints and require dual porting to allow access by both a data processing unit and a video controller, significant cost savings can be achieved by avoiding the duplication of memory control hardware.
One major potential problem with the use of a single memory array for multiple display stations is that the required speed of access to the memory array could exceed the speed of operation available on current low cost memory devices. A second potential problem is the difficultly in devising an efficient and convenient memory addressing scheme for use by the data processing portion of the system. These potential problems are overcome by the present invention.
Another aspect of the present invention is that the same memory system can be used either for multiple monochrome display stations, or for a smaller number of color display stations. In accordance with this aspect of the present invention, the memory system has two modes of operation: one mode for multiple monochrome stations and one mode for color displays. This aspect of the present invention can produce significant cost savings by eliminating the need for separate monochrome and color system designs and also eliminating the corresponding duplication in inventory and quality control expenses.
It is therefore a primary object of the present invention to provide an improved video memory system for computer systems having a plurality of display stations.